This application relates to the field of digital imaging. It is more specifically concerned with the insertion and detection of an identifying mark on a work-piece.
It is a constant endeavor to find improved techniques of placing a visible or invisible identifying mark on an image. This is generally useful to establish ownership, origin and authenticity, and also to discourage those who might wish to purloin or misappropriate the work. Identifying marks are also useful to give evidence of unauthorized alteration or disclosure.
Visible marks are herein classified as being either visible robust or visible fragile. A mark is classified as visible robust it it can be seen by the unaided eye and cannot be easily removed from the work-piece, if at all, without leaving telltale evidence. It is classified as visible fragile if the mark itself is visibly altered by an attempt to alter the work-piece or its wrapper.
Invisible marks are herein classified relative to the appearance of that mark to a human being with normal visual acuity. A mark on an image is classified as having an invisibility classification level of undetectably invisible if, when the imaqe without the marking is displayed together with an image copy with the marking, the human being is equally likely to select either of these copies. An undetectably invisible mark is below or at the human being""s just noticeable difference. A mark on an image is classified as having an invisibility classification level of subliminally invisible if the mark is not distracting to the human being, although it is above the human being""s lust noticeable difference. An image marking is classified as being marginally invisible if it does not cause the marked image to lose its usefulness or value because of the mark. An image marking is classified as being poorly invisible if the marking causes a reduction in the image""s usefulness and/or value.
Presently, both visible and invisible markings of hardcopy documents are used as a generally dependable method of establishing ownership and authenticity. These time-tested methods are also useful for marking a xe2x80x9csoftcopyxe2x80x9d digitized image, also referred to herein as a digital image or image. A digital image is an abstraction of a physical image that bag been scanned and stored in a computer""s memory as rectangular arrays of numbers corresponding to that image""s (one or more) color planes. Each array element corresponding to a very small area of the physical image is called a picture element, or pixel. The numeric value associated with each pixel for a monochrome image represents the magnitude of its average brightness on its single color (black and white) plane. For a color image, each pixel has values associated and representing the magnitudes or average brightnesses of its tristimulus color components representing its three color planes. Other image representations have more than three color components for each pixel. A different value is associated with each different one of the image""s color planes.
In what follows, whenever reference is made to color planes it is understood to include any number of color planes used by a particular image""s digitizing technique to define the pixels color characteristics. This includes the case when there is only a single plane defining a monochromatic image.
A digitalized image is recognizable as an image to a viewer only when the individual pixels are displayed as dots of white or colored light on a display or as dots of black or colored inks or dyes on a hardcopy. Pixels are normally spaced so closely as to be unresolvable by a human visual system. This results in the fusion of neighboring pixels by the human visual system into a representation of the original physical image. Image fusion by the human visual system makes invisible marking, or relatively invisible marking, of images possible. This property is fully exploited by the methods described here to both impart upon a digitized image an invisible watermark to a desired invisibility classification, and to subsequently demonstrate its existence. The imparting and demonstrated detection of a robust invisible marking on digital images, herein called invisible watermarking, are a primary aspect of the present invention.
A proper invisible watermarking technique that imparts an invisible watermark upon a proprietary digitized image should satisfy several properties. The imparted watermark should appear to be invisible to any person having normal or corrected visual accomodation to a desired invisibility classification level. Clearly, the degree of marking is a dichotomy. A balance has to be struck between protecting the image from unauthorized uses and not having the watermark unpleasantly alter the appearance of the image. This generally means that a recognizable pattern should not appear in the marked image when the watermark is applied to a uniform color plane. This requirement discourages marking the image by varying the hue of its pixels since the hunan visual system is significantly more sensitive to alterations in hue than in brightness. The requirement can be satisfied by a technique based on varying pixel brightness implemented in a proper way. A technique based on varying pixel brightness also allows the same marking technique applied to color images to be equally applicable to monochrome images.
Another property of a proper invisible watermarking technique is that it should have a detection scheme such that the probability of a false-positive detection is vanishingly small. For purposes of the present invention, the probability of detection of a watermark in an image when one does not exist should be less than one in a million. There is generally little difficulty satisfying this requirement when the technique is statistically based.
Still another property of a proper watermarking technique is that it should be possible to vary the degree of marking applied to an image. In this way, the watermark can be made as detectable as necessary by the particular application. This property is important in highly textured images where it is often necessary to increase the intensity of the mark to increase its likelihood of detection. This is in contradiction with images that have low contrast in which it is advantageous to reduce the marking intensity to lessen undesirable visible artifacts of the watermark itself.
It is also highly desirable that when detected the demonstrated existence of the watermark should be translatable to a recognizable visual image having relatively bold features with a high contrast ratio. Features of a demonstrated visual image that are not relatively bold may otherwise be difficult to show if the watermark has been attacked in attempts to defeat its protection.
Finally, the imparted watermark should be robust in that it should be very difficult to be removed or rendered undetectable. It should survive such image manipulations that in themselves do not damage the image beyond usability. This includes, but is not limited to, JPEG xe2x80x9clossyxe2x80x9d compression, image rotation, linear or nonlinear resizing, brightening, sharpening, xe2x80x9cdespeckling,xe2x80x9d pixel editing, and the superposition of a correlated or uncorrelated noise field upon the image. Attempts to defeat or remove the watermark should be generally more laborious and costly than purchasing rights to use the image. It the image is of rare value, it is desirable that the watermark be so difficult to remove that telltale traces of it can almost always be recovered.
An aspect of the present invention is to provide a method for imparting a watemiark onto a digitized image comprising the steps of providing the digitized image, arid multiplying the brightness data associated with at least one of the image pixels by a predetermined brightness multiplying factor. The image includes a plurality of pixels, wherein each of the pixels includes brightness data that represents one brightness value if the image is monochrome, or a plurality of brightness data values if the image has multiple colors. In an embodiment the brightness multiplying factor ranges from 0.9 to 1.1. The brightness multiplying factor has a relationship with a number taken from a random number sequence and the relationship is a linear remapping to provide a desired modulation strength.
In an embodiment, each of the pixels has a row and a column location in an array representing the digitized image, and the brightness multiplying factor employs a different sequential combination of numbers from a robust random number sequence in sequential correspondence to the row and column location.
Another aspect of the present invention is to provide a method for generating a watermarked image wherein a watermark is imparted onto a digitized image having a plurality of original elements having original brightnesses. The method includes the step of providing a digitized watermarking plane comprising a plurality of watermarking elements, having a watermark brightness multiplying factor and having one-to-one correspondence with the original elements. It also includes the step of producing a watermarked image by multiplying the original brightness of each of the original elements by the brightness multiplying factor of a corresponding one of the watermark elements wherein the watermark is invisible. In an embodiment, the original image forms an original plane and the watermarking image forms a watermarking plane being smaller than the original plane, the method further includes the step of extending the watermarking plane by tiling such that the watermarking plane covers the original plane and/or further comprises the step of truncating the watermarking plane such that the watermarking plane covers the original plane, upon determining that the watermarking plane extends beyond tile original plane.
Another aspect of the present invention is to provide a method for forming a watermarking plane including a plurality of elenents each having a multiplying value. The method comprises the steps of: generating a secure random sequence of integers having a first plufality of bits; linearly remapping the random sequence to form a remapped sequence of brightness multiplying factors to provide a desired modulation strength; computing a discrete Fourier transform of the remapped sequence to tori, a courier sequence having ftequency coordinates; expanding the frequency coordinates to form an expanded sequence; and computing an inverse Fourier transform of rhe expanded sequence to obtain a watermarking seguence of values.
An embodiment further includes one or more or the following: the step of expanding is accomplished by zero-padding; the method further comprises a step of employing the watermarking sequence to provide the multiplying value for each of the elements; the method further comprises the steps of hardclipping the watermarking sequence to form a hardclipped sequence having sequence members, and utilizing a different one of the sequence members to provide the multiplying value for each at the elements; the method further comprisesthe steps of adjusting the watermarking sequence to form a normalized sequence of values having a mean and a median equal to the difference between unity and the modulation strength, and having a maximum of unity, and employing tbe normalized sequence to provide the multiplying value for each of the elements; the method further comprises the steps of providing an unmarked original image having a plurality of original pixels, each of the pixels having at least one brightness magnitude, wherein a first number of the original pixels is greater than a second number of the plurality of elements, expanding the watermarking plane by tiling to cover the unmarked original image such that one of each of the pixels has one corresponding element from the elements; and multiplying the at least one brightness magnitude of each of the pixels by the niultiplying value of the corresponding element.
Still another aspect of the present invention is to provide a method for detecting a watermark in a marked image. The marked image is marked by a watermarking plane which has a plurality of watermarking elements. Each at the image pixels has at least one brightness value and each of the watermarking elements has a brightness multiplying factor. The method employs a selector and a visualizer and at least one counter to store the comparison data resulting from comparisons for each of a plurality of selector elements and positions, of the statistical brightness of image pixels and watermarking plane elements with the statistical brightness of neighboring elements/pixels. The method further comprises the step of displaying a visualizer coincidence image such that a user can make a determination as to whether the pattern encoded by the visualizer pixels is recognizable and the watermark is detected.